The invention relates to a contactless conductance potentiometer having two exterior electrodes and a tap for dividing the conductance.
Classic potentiometers consist of an ohmic resistance element formed by a coiled wire or of an extended resistance wire or of a resistance path. The element has two electric connections at the exterior and therebetween a movable sliding contact as the tapping electrode. The sliding contact is activated either via a sliding device or via a rotary device. If one applies a voltage to the exterior electrodes, the tap divides this voltage according to its position on the resistor path. The partial voltages are proportional to the resistance parts lying between tap and exterior electrodes. The total ohmic resistance remains thereby always the same.
If one does not wish to divide a voltage, but rather an electric current, one must divide a conductance which maintains the same in the sum. For this purpose, the classic potentiometer with the linear proportional resistance division is not suited. The sum of the conductance values is not constant. One must divide a resistance path such that a linear proportional conductance division is carried out with the tapping motion.
Independent from this differentiation of a resistance division and a conductance division, classic potentiometers have the disadvantage that noise is caused by the sliding contact and that the life span is delimited by the wear and tear of the contact.
The tap can be designed without contact in order to prevent this. Either the sliding contact is replaced by a different type of contacting means, for example, by exposing the desired tap location in a photoconductor rail which then produces the connection of the point in the resistance- or conductance-value element, corresponding with the point to be contacted, with the tapping electrode, or one alters the resistance value of two partial elements with a fixed tap connection by means of exterior influence, for example, by means of a magnetic field in magnetic field-dependent resistance elements.
In the first case, the disadvantage exists that the activated portion of the photoconductor layer which produces the contact to the tapping electrode has itself a resistance value. In the second case, linearity is difficult to produce, in particular, when an electronic circuit connected to the subjective divider arrangement is required in order to divide the desired electric magnitude.